Accretion Disc Electrodynamics -- A Model for Double Radio Sources

A mechanism is proposed that may account for the production of two oppositely-directed beams of relativistic plasma such as have been inferred to be the energy supply of strong, double radio sources. High angular momentum material accreted by a compact object can settle into a stationary disc. If this material carries with it a vertical component of magnetic field (i.e. parallel to the rotation axis) then a magnetosphere can form above and below the disc similar in character to that found in an axisymmetric model of a pulsar. An exact analytic solution is given describing a force-free magnetosphere with this geometry. This solution has the property that energy and angular momentum are extracted in such a way that the disc can evolve in a stationary fashion, without invoking additional viscous torques. It may also provide some qualitative insight into the more difficult problem of the electromagnetic structure of the open field lines of a pulsar. In the application to double radio sources such a machine could operate within the nucleus of an associated galaxy or quasar. In this case the two beams would be produced with a flow velocity which is already supersonic, thus evading some of the stability problems associated with models in which the flow is initially subsonic. This proposed explanation may also be appropriate for the double source positionally associated with the X-ray object Sco X-1.